We always use the terms "small voice" and "big voice" because that is more concrete. There are times when you might be outside but still need an quiet voice which leads to confusion. Other than modeling and practice there are not a lot of options. Have you had his hearing checked or could he have fluid in his ears from allergies? That will cause that loud talking too. You could try playing a game where you use a voice and then ask him if it was small or big and then ask him to model similar voices for you. At three the most effective thing I have found is to catch them doing what you want and praise the heck out of them.